1. Field of the Invention
The present invention relates to a Group III-V compound semiconductor crystal structure, and a method of epitaxial growth of a Group III-V compound semiconductor crystal structure, as well as a semiconductor device over a Group III-V compound semiconductor substrate.
2. Description of the Related Art
It is preferable to grow an epitaxial layer on a base material or a substrate which comprises the same bulk semiconductor crystal as the epitaxial layer, wherein the base material is identical with the epitaxial layer in physical properties such as lattice constant and thermal expansion coefficient. This results in a reduced crystal defect of the epitaxial layer and also improves the quality of the epitaxial layer.
A gallium nitride based bulk crystal has a high dissociation pressure of nitrogen, for which reason it is generally difficult to prepare a large size wafer of the gallium nitride based bulk crystal. The gallium nitride based bulk crystal generally has a low electron mobility in the range of about 30-90 cm2/Vs due to a relatively high impurity concentration.
The present inventors had prepared the gallium nitride based semiconductor substrate in a conventional method which is disclosed in Japanese laid-open patent publication No. 11-251253. FIGS. 1A through 1E are fragmentary cross sectional elevation views of semiconductor substrates including gallium nitride base layers in sequential steps involved in the conventional methods.
With reference to FIG. 1A, a GaN film 112 having a thickness of 1.2 micrometers is formed on a (0001)-face of a sapphire substrate 111. An SiO2 film is formed on a surface of the GaN film 112. A photo-lithography process and a subsequent wet etching to the SiO2 film are carried out for forming silicon oxide masks 114. The silicon oxide masks 114 cover parts of the surface of the GaN film 112. Uncovered parts of the surface of the GaN film 112 are growth areas 113. The growth areas 113 are stripe-shaped in a width of 5 micrometers. The silicon oxide masks 114 are also stripe-shaped in a width of 2 micrometers. The stripe-shaped growth areas 113 and silicon oxide masks 114 have a longitudinal direction  less than 11-20 greater than .
With reference to FIG. 1B, a GaN film 115 is grown on the growth areas 113 by a hydride vapor phase epitaxy (HVPE) which uses gallium chloride (GaCl) and ammonium (NH3) as Group-V source material. Gallium chloride (GaCl) is produced by reaction of gallium as group-III element, and hydrogen chloride (HCl). Dichlorosilane (SiH2Cl2) is used as an n-type dopant. The substrate 111 is set in a growth chamber. A temperature is increased up to a growth temperature of 1000xc2x0 C. in a hydrogen atmosphere. After the temperature becomes stable at the growth temperature, then hydrogen chloride (HCl) is supplied at a flow rate of 20 cc/min for 5 minuets, whereby GaN films 115 are grown on the growth areas 113, wherein the GaN films 115 have a (1-101)-facet structure.
With reference to FIGS. 1C through 1E, feeding dichlorosilane (SiH2Cl2) as the n-type dopant is still continued to further grow up the GaN films 115 until a thickness of the GaN films 115 becomes 100 micrometers. The crystal growth based on the (1-101)-facet makes the GaN films 115 grown over the masks 114 in the lateral direction. This is so called as an epitaxial lateral overgrowth. This growth method allows obtaining a crack-free wafer of 2-inches size including a GaN film of a few hundreds thickness. The obtained wafer has a reduced dislocation density and a doping concentration of 5E17/cm3 and a mobility of about 800 cm2/V.
Even the dislocation density is reduced by using the epitaxial lateral overgrowth but still be about 1E7/cm3. This dislocation density means that the GaN film has a prismatic-shaped micro crystal structure including a large number of prismatic-shaped micro-crystal grains. This prismatic-shaped micro crystal structure has a tilt toward an c-axis of the crystal structure and a twist in a c-plane thereof. For this reason, the GaN film has a mosaic crystal structure which may generally cause deterioration in electrical and optical properties.
The tilt angle and the twist angle may be measured by using an X-ray. If a single GaN film having a thickness of 1.2 micrometers is grown on a low temperature buffer layer over a sapphire substrate without using the epitaxial lateral overgrowth, then the measured tilt angle of the GaN film is 324 seconds, and the measured twist angle is 1188 seconds. If a GaN film having a thickness of 140 micrometers is grown on the sapphire substrate by using the epitaxial lateral overgrowth, then the measured tilt angle of the GaN film is 180 seconds, and the measured twist angle is 208 seconds.
The use of the epitaxial lateral overgrowth results in a certain improvement in the crystal quality of the gallium nitride based layer. Notwithstanding, the tilt angle and the twist angle of the crystal structure of the gallium nitride based semiconductor layer are still larger than when the gallium nitride based layer is grown over a GaAs substrate or an InP substrate.
For further improvement in the electrical and optical characteristics of the semiconductor device, it is desirable to further reduce the tilt angle and the twist angle of the crystal structure of the gallium nitride based semiconductor layer.
Accordingly, it is an object of the present invention to provide a novel method of forming a Group III-V compound semiconductor layer free from the above problems.
It is a further object of the present invention to provide a novel method of forming a Group III-V compound semiconductor layer with a highly perfection of crystal structure.
It is a still further object of the present invention to provide a novel Group III-V compound semiconductor layer free from the above problems.
It is yet a further object of the present invention to provide a novel Group III-V compound semiconductor layer with a highly perfection of crystal structure.
It is a still further object of the present invention to provide a novel semiconductor device including a Group III-V compound semiconductor layer free from the above problems.
It is yet a further object of the present invention to provide a novel semiconductor device including a Group III-V compound semiconductor layer with a highly perfection of crystal structure.
It is a still further object of the present invention to provide a novel semiconductor substrate with an upper surface comprising a Group III-V compound semiconductor layer free from the above problems.
It is yet a further object of the present invention to provide a novel semiconductor substrate with an upper surface comprising a Group III-V compound semiconductor layer with a highly perfection of crystal structure.
The present invention provides a method of forming a Group III-V compound semiconductor layer. The method comprises the step of: carrying out an epitaxial growth of a Group III-V compound semiconductor layer over a base layer having a crystal structure by use of a mask, wherein the mask satisfies the equation (1):
hxe2x89xa7(w/2)tan xcex8xe2x80x83xe2x80x83(1) 
where xe2x80x9cxcex8xe2x80x9d is a base angle of a facet structure of the Group III-V compound semiconductor layer on the epitaxial growth; xe2x80x9chxe2x80x9d is a thickness of the mask; and xe2x80x9cwxe2x80x9d is an opening width of the mask at its lower level, and the opening width is defined in a direction included in a plane which is vertical to both the surface of the base layer and the side face of the facet structure.
The present invention also provides a Group III-V compound semiconductor epitaxial layer having an upper region which has a crystal structure with a tilt angle of at most 100 seconds and/or a tilt angle of at most 100 seconds.
The above and other objects, features and advantages of the present invention will be apparent from the following descriptions.